Along with the widespread use of the Internet and cell phones, carrier networks are shifting from time a division multiplexing (TDM) network, such as related art synchronous optical network/synchronous digital hierarchy (SONET/SDH), to a packet-based network, such as Ethernet (registered trademark) (hereinafter simply referred to as local area network (LAN)), employing LAN technique and Internet Protocol (IP) technique.
A communication company limits communication traffic to a contracted rate below which each user is permitted to transmit information to a network. To this end, the communication company requests a communication apparatus at a packet-based network of the communication company to control a transmission frame rate. The frame rate herein refers to an amount of data in a quantitative sense represented by unit, such as bit per second (bps).
In the packet-based network, in-frame information is referenced as an identifier to identify a user. The in-frame information includes virtual local area network (VLAN) tag, media access control (MAC) address, Internet protocol (IP) address, and multiprotocol label switching (MPLS) label. The identifier is not only checked against a reception frame and a transmission frame in an apparatus, but also newly attached to or deleted from the frame in the apparatus.
In such a case, a frame length of the reception frame or the transmission frame may be dynamically modified at a time point of input to or at a time point of output from the communication apparatus. Rate control of the transmission frame requested of the communication apparatus is performed so that frames externally transmitted from the communication apparatus, including a frame dynamically modified in frame length, are to be transmitted at a set upper rate or below.
FIG. 1 illustrates a configuration of an example of a network. Communication apparatuses 1a through 1f are mutually connected and form a network. The communication apparatus 1a is connected to user terminals 2a through 2i, and the communication apparatus 1d is connected to user terminals 3a through 3j. The communication terminal 1c is connected to user terminals 4a through 4k, and the communication terminal 1f is connected to user terminals 5a through 5m. 
Each communication terminal receives a frame transmitted from a user terminal, and transfers the frame to an intended destination in accordance with address information or the like stored within the frame. For example, when the user terminal 2a transfers a LAN frame or an IP frame, the communication apparatus is performs a transfer process on the frame based on an MAC address stored in the LAN frame or an IP address stored in the IP frame. In the following discussion, a communication 1 represents each of the communication apparatuses 1a through 1f. 
FIG. 2 is a block diagram of an example of the communication apparatus 1. The communication apparatus 1 includes network interface (IF) circuits 11-1 through 11-n, switch circuit 12, and control circuit 13. In the following discussion, a network IF circuit 11 represents each of the network IF circuits 11-1 through 11-n. As illustrated in FIG. 2, an arrow-headed solid line denotes a data line, and an arrow-headed broken line denotes a control line. Each network IF circuit 11 includes a network port (hereinafter simply referred to as “port”), and performs an interface process, a reception frame process, a transmission frame process and the like in conjunction with an external apparatus. The switch circuit 12 is connected to each network IF circuit 11 via the data line in the communication apparatus 1, and performs a switching process of a transfer frame in conjunction with the network IF circuit 11.
The control circuit 13 is connected to each network IF circuit 11 and the switch circuit 12 via the control lines in the communication apparatus 1, and controls a variety of settings of the network IF circuit 11 and the switch circuit 12 in the communication apparatus 1, and controls alarming, and collection of statistical information. The control circuit 13 is also connected to a control terminal 14 external to the communication apparatus 1. Each of the network IF circuits 11, the switch circuit 12, and the control circuit 13 may be supplied as a detachably mountable module or card, but optionally may be integrated with a mother board (mother card) of the communication apparatus 1.
FIGS. 3A and 3B illustrate formats of LAN frames. FIG. 3A illustrates a frame without a VLAN tag. FIG. 3B illustrates a frame with a VLAN tag. As illustrated in FIGS. 3A and 3B, MAC DA stands for a destination MAC address. MAC SA stands for a source MAC address. E-TYPE indicates a message type to be stored in a subsequent protocol data unit (PDU). For example, 0x0800 (0x represents a hexadecimal notation) indicates an Internet protocol version 4 (IPv4) frame. A message of an upper layer, such as IPv4 frame, is stored in PDU. Frame check sequence (FCS) is CRC-32 code for frame error detection.
Tag protocol ID (TPID) of FIG. 3 is an identifier that indicates that a virtual local area network ID (VLAN ID) is stored in a subsequent field, and typically the identifier is 0x8100 defined by IEEE802.1Q. VLAN ID is a VLAN ID value that identifies a user. TPID and VLAN ID are collectively referred to as a VLAN tag. A plurality of VLAN tags may be consecutively stacked.
Japanese Laid-open Patent Publication No. 2007-214779 discloses a data transfer technique. According to the disclosed data transfer technique, transfer type information is obtained by analyzing received communication data, a frame length of a transmission frame is predicted based on the transfer type information when the transmission frame is transferred, and a transmission band of communication data to be transferred is controlled in accordance with the predicted transmission frame length.